A small motor, which is a so called spindle motor in HDDs must rotate with high precision in order to read, write, and fetch information. Recently, high accuracy is required particularly in positioning in both the radial and thrust directions because of high record densities. Furthermore, for the requirement of energy saving along with down-sizing the HDD, it is also important to reduce the power consumption of the motor.
Conventionally, ball bearings have been extensively used as the motor bearing described above. However, because of the requirement of highly accurate rotation, ball bearings have been gradually replaced with lubricated dynamic pressure oil type bearings. Dynamic pressure oil bearing motors, which utilize oil, are often influenced by the oil properties. This is particularly concerned with increased power consumption at low temperatures and oil leakage and deterioration at high rotation rates. To resolve these problems, it is suggested to use gas in place of lubricating oil in dynamic pressure bearings. For example, when air is used, no problems occur in relation to increased power consumption and lubricant leakage and deterioration. However, because the viscosity of the air is in the order of thousandth of that of the oil, dynamic pressure has to be generated with a larger diameter in order to obtain a rigid bearing that bears an equivalent load to its oil counterpart.
In order to make a dynamic pressure gas bearing as large as possible in the limited space of the motor rotation body, a construction is proposed in which a motor rotation body is supported by a radial dynamic pressure gas bearing formed by the inner surface of the motor rotation body and the outer surface of a stepped cylindrical support shaft and also supported by a thrust dynamic pressure gas bearing formed by the top surface of the support shaft and the bottom surface of the insert of the motor rotation body (reference Japanese Laid Open Patent Publication 2000-50568).
On the other hand, in order to give the radial dynamic pressure gas bearing an extended length, an inner rotor construction is proposed in which a rotor magnet is provided on the outer periphery of a disk receiving part of the motor rotation body with a specified clearance from a set of radial stator coils concentric with the support shaft (reference Japanese Laid Open Patent Publication 2001-352726).